To meet the demand for wireless data traffic, which has increased since deployment of 4th-Generation (4G) communication systems, efforts have been made to develop an improved 5th-Generation (5G) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘Beyond 4G Network’ or a ‘Post Long-Term Evolution (LTE) System’.
It is considered that the 5G communication system will be implemented in millimeter Wave (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To reduce propagation loss of radio waves and increase a transmission distance, a beam forming technique, a massive multiple-input multiple-output (MIMO) technique, a Full Dimensional MIMO (FD-MIMO) technique, an array antenna technique, an analog beam forming technique, and a large scale antenna technique are discussed in 5G communication systems.
In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud Radio Access Networks (RANs), ultra-dense networks, a device-to-device (D2D) communication, a wireless backhaul, a moving network, a cooperative communication, Coordinated Multi-Points (CoMP), reception-end interference cancellation, and the like.
In the 5G system, a Hybrid Frequency Shift Keying (FSK) and Quadrature Amplitude Modulation (QAM) Modulation (FQAM) and a Sliding Window Superposition Coding (SWSC) as an Advanced Coding Modulation (ACM) scheme, and a Filter Bank Multi Carrier (FBMC) scheme, a Non-Orthogonal Multiple Access (NOMA) scheme, and a Sparse Code Multiple Access (SCMA) scheme as an advanced access technology have been developed.
With the rapid development of the information industry, the wireless communication technology is faced with unprecedented challenges in the future. In the foreseeable future, the need for wireless data services in the world will keep high growth. According to a report from the international telecommunication union (ITU), wireless data services will increase at least 1000 times by 2020. In particular countries and regions, the number may be even higher. In response to the unprecedented challenges, 5G communication technologies research has been extensively launched across the world. The enhancement of spectrum efficiency as well as the new allocation of frequency resources and the increase of frequency reusing factor are widely considered as the crux of the 1000 times service boosting.
Among various potential technologies capable of enhancing spectrum efficiency, the distributed antenna technology is proved as a practical and effective method. Compared with a centralized multi-antenna system, the distributed antenna system may flexibly achieve the cooperative transmission between multiple antennas via the multiple antennas deployed on different spatial positions. For example, the distributed antenna system may coordinate the interference between multiple users utilizing cooperative beam-forming. Therefore, the distributed antenna system may achieve a higher peak transmission rate, smarter interference management and a more reliable cell-edge data transmission. Based on above mentioned advantages, the distributed antenna system is gradually introduced in the latest wireless communication standards, such as the coordinated multipoint transmission and reception (CoMP) method in the long term evolution (LTE) system corresponding to the evolved universal terrestrial radio access (E-UTRA) protocol defined by the 3rd generation partnership project (3GPP).
Although being capable of better increasing the spectrum efficiency of the communication system, the distributed antenna system also raises more rigorous implementation conditions during the practical deployment process [1]. In order to achieve some multi-antenna cooperative transmission, such as joint processing (JP), it is necessary for the multiple antennas to maintain a high synchronization precision, so as to ensure that signals transmitted from different antennas can maintain a better time-frequency consistency at the receiving end, that is, signals transmitted from multiple antennas should arrive at antennas of the receiving end with a minimum time difference, and carrier frequency offsets between the multiple signals should be as small as possible. In order to obtain the time-frequency consistency, the existing system will define a corresponding requirement standard regarding devices and deployment based on the signal frame structure used by the system, so as to guarantee that the system performance will not have a significant loss due to the imperfect time-frequency offset. For instance, in an LTE system based on the orthogonal frequency division multiplexing (OFDM) modulation, the time difference of arrival (TDOA) of signals transmitted from different antennas may be restricted to the length of a cyclic prefix (CP). That is to say, the sum of an antenna time delay and a transmission time delay is less than the length of the CP.
However, in the future communication system, it may be more difficult to satisfy the rigid time-frequency synchronization requirement. On one hand, with the cell densification, the rigid time-frequency synchronization will bring high deployment costs due to the large scale demand for large-bandwidth and low-latency fiber-optic backbone network. One the other hand, the short sub-frame structure [2] possibly used by the air-interface in the future communication system will severely limit the application of the distributed antenna system. For example, because the high frequency communication which has attracted more and more attention adopts higher frequency-band, the OFDM system used by the communication system will become more sensitive to the Doppler shift. Therefore, it is necessary to shorten the length of the signal to obtain larger carrier spacing. For example, for a high frequency communication system using 28 GHz, the carrier spacing thereof is designed to be 270 kHz to accommodate the Doppler shift caused by mobile communication. Therefore, the length of the OFDM symbol used by the system is 3.70 μs, and the length of the CP is 0.46 μs. When the distributed antenna system is used under such frame structure, even if the time for transmitting signals from multiple antennas is strictly synchronous, the difference between distances from the user to each of multiple antennas will result in serious time dis-synchronization. For instance, when the difference between two distances from a user to one antenna and from the user to another antenna is 140 m, the TDOA of signals transmitted from the two antennas will be larger than the length of the CP (0.47 μs). When the multi-path effect is considered, the TDOA passing through channels of multiple paths will become larger. Since the time difference is larger than the length of the CP, the received multipath signals will suffer serious inter-carrier interference (ICI) and inter-symbol interference (ISI), thus the reliability of signal reception will be degraded greatly. The high frequency communication is only one scenario of the short sub-frame, and the low frequency communication may also use the short sub-frame to reduce the transmission delay [2]. In addition, even if a longer sub-frame structure is used in the low frequency communication, the time difference of receiving signals may be too large resulted from the time delay of the backbone network and imperfect components. On the other hand, although the usage of the CP simply solves the problem of channel delay spread, the CP also reduces the spectrum efficiency of the system. Due to above mentioned time delay difference of the distributed antenna system, the system may need a longer CP to guarantee the reliability of signal reception, which will no doubt further reduce the spectrum efficiency of the system.